Packet framer

ABSTRACT

A frame layer communications system for packet-switched virtual circuits that provides error detection when the link layer both corrupts and discards cells. An illustrative embodiment of the invention provides two error detection mechanisms: the first for the user-data and the second for certain control information. When the frame is segmented into cells, the control information and information relating to the second error detection mechanism are segmented into a single cell.

This application is a continuation of application Ser. No. 08/349,148,filed on Dec. 2, 1994, now abandoned, which is a continuation ofapplication Ser. No. 07/836,030, filed on Feb. 14, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to communications systems in general, and, moreparticularly, to methods and apparatus for error control incommunications systems which employ virtual circuit technology.

2. Background of the Invention

A communications system permits users to communicate over an extendeddistance and typically comprises a multiplicity of devices andinformation channels to facilitate the communication. Perhaps the mostfamiliar and ubiquitous of all communications systems is the telephonesystem. This includes, for example, the local- and long-distancetelephone networks, private networks and the private branch exchangesused to provide special services to businesses and institutions. Whilethe telephone system carries information in both voice and data forms,similar computer networks carry data between computers.

FIG. 1 presents an illustrative communication system comprising acommunication network 109 connecting two users, Alice 101 and Bob 103.When Alice desires to transmit information to Bob, she provides hersystem station 105 with the information. The system station may be anydevice (e.g., a telephone, facsimile machine or computer) which iscapable of transforming the information a user provides to a form thecommunication system can handle, and vice versa. When the system stationis an analog telephone, the system station transforms the acousticvibrations of Alice's voice into an ordered set of signals representinganalog quantities ("analog signals") for transmission over thecommunication system. When the system station is a digital telephone,facsimile machine or computer, the system station typically transformsthe information into an ordered set of signals or "packet" representinga set of digital quantities ("digital signals").

After the system station 105 creates the set of signals, the systemstation transmits the signals, over a communication network 109, toBob's system station 107 where Bob's system station transforms thesignal into a form that is suitable for presentation to Bob.

A typical packet switching system is disclosed in U.S. Pat. No. Re.31,319, by A. G. Fraser, reissued Jul. 19, 1983, and enabledcommunications systems to carry packets. Such packet-switchedcommunications systems are well known in the prior art, carry both voiceand non-voice data and generally are more economical and utile thantheir analog counterparts.

A packets switched communication system can destroy a packet either bycorrupting (i.e., changing the signals in) it or by losing all or aportion of the signals in it. A packet retains its "integrity" when itis delivered with no changed or lost signals. When a packet is destroyedduring transmission, however, it becomes impossible to determine theintegrity packet. Although merely determining whether a packet has beendestroyed is problematic, once it is determined that the packet has beendestroyed, the generally accepted procedure is to discard the packet andrequest retransmission by the sender. This is the practice followed bythe TCP/IP protocol suite and such other well-known communicationsprotocols as LAP/LAPB/LAPD, X.25/X.75 and Q.921/Q.931.

To determine whether a packet has been destroyed it is necessary to makecertain assumptions about how the packet can be destroyed. When it canbe assumed that the signals within a packet may be changed but neverlost, a Cyclic-Redundancy-Check will determine whether the integrity ofthe packet has been destroyed or not. These are the assumptions and thesolution for many data networks (e.g., X.25, Ethernet, FDDI and Bisync).

When it can be assumed that the signals within the packet can be lostbut not changed, a packet length indicator and a packet sequenceidentifier will determine whether the integrity of the packet has beendestroyed or not. These are the assumptions and the solution used inDatakit networks marketed by AT&T, as implemented in the UniversalReceiver Protocol disclosed in Fraser et al, U.S. Pat. No. 4,852,127,issued Jul. 25, 1989.

Packet-switching networks which employ "virtual circuit" technologyallocate network resources such that all of the information sent acrossthe network is sent successively in the order that the information issubmitted to the network. Such virtual circuit packet-switching networksare well-known in the prior art, generally carry non-voice data and areutile than their datagram counterparts.

Virtual circuit packet-switching networks can both lose signals within apacket and can corrupt the remainder and therefore it is not possible tomake any of the simplifying assumptions made for networks above. Thus,it has been problematic to devise a mechanism for determining whetherthe integrity of a packet has been destroyed or not in virtual circuits.Merely combining the mechanisms of a Cyclic-Redundancy-Check, a packetlength indicator and a packet sequence identifier is insufficient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a map of an illustrative communications network whichprovides communications facilities between two parties.

FIG. 2 presents an illustrative embodiment of the present inventionwhich provides error detection in a communications system.

FIG. 3 presents an illustrative frame containing user-data and otheraccounting information.

FIG. 4 presents a detailed description of the functionality provided bythe packet encapsulator module in FIG. 2.

FIG. 5 presents a derailed description of the functionality provided bythe frame segmentor module in FIG. 2.

FIG. 6 presents a derailed description of the functionality provided bythe trailer validator module in FIG. 3.

FIG. 7 presents a derailed description of the functionality provided bythe packet validator module in FIG. 3.

SUMMARY OF THE INVENTION

The present invention avoids the failings of the prior art whileproviding a mechanism for detecting both the destruction and corruptionof an ordered set of signals transmitted over a Lossy communicationchannel. These results are obtained in an illustrative embodiment of thepresent invention which receives as input an ordered set of signals tobe communicated and augments the set of signals with error detection andother control signals. All of these signals are then segmented into atleast one set of output signals such that some of the error detectionsignals and the accounting signals are contained within a single set ofoutput signals. Embodiments of the present invention are more efficient(i.e., have less overhead) than solutions in the prior art and are moreeasily implemented.

DETAILED DESCRIPTION 1. Introduction

For pedagogical reasons, illustrative embodiments of the presentinvention are presented in the context that one user, Alice, desires toreliably transmit a voice or non-voice data message to a second user,Bob, over a communication system which employs virtual circuittechnology. While an attempt is made by the virtual circuit to assurethe validity of the packet, in fact, the packet either may be eithercorrupted or not delivered at all.

It should be noted, however, that a salient characteristic of virtualcircuit technology is that while the virtual circuit may either corruptthe packet, deliver only parts of it, or not deliver it at all, thoseparts that are delivered are delivered in the order sent. This isbecause the same physical (or logical) path is typically used for all ofthe signals in the packet. A feature of the illustrative embodimentadvantageously provides a mechanism for detecting the corruption ordeletion of the packet, or parts of the packet. Error correction,usually in the form of requests for packet retransmission, areadvantageously not handled by the embodiment but are left to Bob (thatis, the embodiment of the present invention is used in conjunction withstandard error correction techniques to fully accomplish high accuracydata communications).

Attached as Appendices 1 and 2 are listings of illustrative computerprograms written in the well-known C language corresponding to thefunctions described below. When executed on any one of a large class ofgeneral purpose computers, these programs will realize hardwareembodiments of the present invention. Appendices 3 and 4 are tables ofdata which are used by the computer programs listed in Appendices 1 and2. Appendix 5 is a listing of a computer program which generated thetables in Appendices 3 and 4.

In the following, each reference to a "byte" refers to an 8-bit byte. Itwill be clear to persons having ordinary skill in the datacommunications field how to modify the illustrative embodiments tooperate on other than 8-bit bytes.

2. An Apparatus for Framing a Packet

As shown at 205 in FIG. 2, an illustrative embodiment of the presentinvention advantageously comprises three modules: (1) a packetencapsulator 211 for encapsulating the packet to create a "frame", (2) aframe segmentor 213 for segmenting the frame into one or more "cells",and (3) a cell transmitter 215 for transmitting to Bob the cell or cellsover the virtual circuit 225. For pedagogical reasons each module isdescribed below in terms of the functionality that it provides. It willbe clear to persons of ordinary skill in the data communications fieldhow to fabricate each module with either special purpose hardware orwith appropriately programmed general purpose hardware. It is preferredthat the illustrative embodiment be constructed with either electrical,optical or electro-optical technologies. It will be clear to persons ofordinary skill how these technologies may advantageously be used topractice the instant invention or so much of it as is of use. Inparticular, systems of the type described in the above referenced U.S.Pat. No. Re. 31,319, will be found by those skilled in the: art to bereadily adaptable for embodiments of the present invention.

The embodiment in FIG. 2 receives as input from Alice 201 an ordered setof signals called a "packet." The packet is the user input to theembodiment and may be thought of as the data payload. There are norestrictions on the contents or the structure of the data in the packetand it may advantageously have any length (but lengths of between zeroand 65,535 (2¹⁶ -1) bytes are preferred for this particular embodiment).

The first module 211 in the embodiment encapsulates the packet to createan augmented set of ordered signals known as a "frame." As shown in FIG.3, a frame is a variable-length set of signals that advantageouslycomprises, in order, the following six fields: (1) a "user-data" field,(2) a "padding" field, (3) a "CRC-data" field, (4) a"sequence-identifier" field, (5) a "user-data-length" field and (6) a"CRC-trailer" field.

As shown at 411 in FIG. 4, the module for encapsulating the packetadvantageously performs at least six steps: (1) stuffing the paddingfield, (2) generating the CRC checksum for the CRC-data field, (3)developing a sequence-identifier for the sequence-identifier field, (4)developing the length of the user-data field to store in theuser-data-length field, (5) generating the CRC checksum for theCRC-trailer field, and (6) concatenating the user-data field, thepadding field, the CRC-data field, the sequence-identifier field, theuser-data-length field and the CRC-trailer field, in order, to createthe frame. It should be understood that the order of the fields within aframe is not critical and the described embodiment may be readilymodified to accommodate variations in the order of the fields in theframe.

The user-data field advantageously includes the packet (i.e., themessage that Alice desires to convey to Bob). The information in thepacket is advantageously put into the user-data field withouttranslation, modification, or adjustment and, therefore, there is aone-to-one correspondence between the information in the packet and theinformation in the user-data field. Because the packet may have anarbitrary length of up to 65,535 bytes, so may the user-data field.

The padding field is "stuffed" or filled out with between zero and sevenbytes of arbitrary data so that the overall length of the frame isforced to a multiple of 8 bytes. That the preferred frame length ischosen to be a multiple of 8 bytes arises from (1) the fact that, inthis embodiment, the "trailer" (i.e., the sequence-identifier field, theuser-data-length field and the CRC-trailer field) is, in toto, 8 byteslong, and (2) the general requirement that the frame be segmented suchthat the trailer be contained within a single cell as will be describedmore fully below. Therefore, when the length of a cell and the length ofthe frame are both multiples of the length of the trailer, it is assuredthat the frame will always be properly segmented. It should be clear tothose skilled in the data communications field how to modify the rangeof bytes in the padding field to accommodate desired modifications tothe embodiment.

In this embodiment, the length of the padding field (i.e., the number ofarbitrary data bytes stuffed into the padding field) is advantageously afunction of the length of the user-data, n, and may be determined asfollows:

    length of padding field=(65,540-n) mod 8

Where "(65,540-n) mod 8" means the remainder when the quantity (65540-n)is divided by 8. For example, when the user-data is 14,386 bytes long,the padding field is to be 2 bytes long.

The CRC-data field is advantageously four bytes long and contains anerror detection string that enables the detection of transmission errorsin the user-data field and padding fields. Specifically, the data in theCRC-data field is advantageously the generated Cyclic-Redundancy-Codechecksum resulting from applying the generator polynomial:

    g(x)=x.sup.32 +x.sup.31 +x.sup.4 +x.sup.3 +x+1

to the data within both the user-data and padding fields. Although theCyclic-Redundancy-Code is the preferred error detection mechanism, itwill be clear to those skilled in the art how to substitute anothererror detection mechanism for the Cyclic-Redundancy-Code mechanism.

The sequence-identifier field is typically three bytes long and containsan identifier of the input sequence or packet. The sequence-identifieris used to identify each frame transmitted between a given transmitter(e.g., Alice) and a given receiver (e.g., Bob). Thus, each transmitter(receiver) must maintain a "synchronized" (i.e., the records of the boththe transmitter and the receiver should be the same) record of thesequence-identifier last transmitted (received) to (from) a givenreceiver (transmitter). In other words, when Alice transmits packets toBob, Carol and David, Alice and Bob must maintain one synchronizedsequence-identifier, Alice and Carol must maintain a second synchronizedsequence-identifier and Alice and David must maintain a thirdsynchronized sequence-identifier.

When a transmitter has not previously transmitted a frame to a givenreceiver, the sequence-identifier for that Transmitter-Receiver pair isadvantageously initialized at zero. Subsequently, thesequence-identifier for that Transmitter-Receiver pair is incremented byone (mod 2²⁴) for every frame that is transmitted from the Transmitterto the Receiver. The sequence-identifier for a frame is not reused whenthe frame is retransmitted; rather, the next regular sequence-identifieris used.

The user-data-length field is advantageously two bytes long andrepresents the length, in bytes, of the user-data field. The CRC-trailerfield is advantageously three bytes long and contains an error detectionstring that enables Bob to detect transmission errors in the trailer.Specifically, the data in the CRC-trailer field is advantageouslydetermined (1) by generating a Cyclic-Redundancy-Code checksum ("CRC")resulting from applying the generator polynomial:

    g(x)=x.sup.23 +x.sup.22 +x.sup.2 +1

to the data within both the sequence-identifier field and theuser-data-length fields, (2) by multiplying the CRC by two and (3) bybit-wise Exclusive-ORing the product by C00005 (hexadecimal). Themodification of the CRC in steps (2) and (3) assures that the checksumis 24 bits in length (three bytes) and always has a non-zero value.Steps (2) and (3) can alternately be represented by the statement(CRC<<1) xC00005. Although this is the preferred error detectionmechanism, it will be clear to persons having ordinary skill in the damcommunications field how to substitute another error detection mechanismfor the Cyclic-Redundancy-Code mechanism.

The second module 213 in FIG. 2 segments the frame into one or morecells. In this embodiment, all cells are, e.g., 48 bytes long. It willbe clear to persons having ordinary skill in the data communicationsfield how to modify the embodiment to accommodate other particular celllengths useful in other applications.

As shown at 513 in FIG. 5, the second module 213 of FIG. 2 in theembodiment advantageously performs at least these steps in segmentingthe frame into one or more cells: (1) filling the frame, if necessary,(2) dividing the frame into cells, and (3) marking each cell as either a"user-data cell" or as a "trailer cell".

When the frame length is not a multiple of 48 bytes, the frame must haveappended to it one or more "fill bytes" to make the total length of theframe a multiple of 48. The fill bytes have a zero value (i.e.,typically are filled with all zeroes).

The step of dividing the frame into one or more cells begins bynumbering the bytes in the frame from 1 to n beginning with the firstbyte in the user-data field and ending with the last fill byte. Thefirst 48 bytes are assigned to the first cell. The next 48 bytes areassigned to the second cell and so forth until the entire frame has beendivided into one or more cells.

The final step in segmenting the frame involves marking each cell toindicate whether it is a "user-data cell" or a "trailer cell". Each cellwhich does not contain the trailer is marked as a user-data cell whilethe last cell of a frame (which does contain the trailer) is marked asthe "trailer cell".

The final module 215 in the embodiment transmits the marked cells, inorder, over the virtual circuit.

3. An Apparatus for Reclaiming a Packet

As shown at 207 in FIG. 2, a receiver or packet reclaiming system inaccordance with an illustrative embodiment of the present inventionadvantageously comprises three modules: (1) a cell receiver 217 forreceiving one or more cells from the virtual circuit 125 to create a"candidate frame", (2) a trailer validator 219 for validating theintegrity of the trailer, and (3) a packet validator 221 for validatingthe integrity of the packet. For pedagogical reasons each module isdescribed below in terms of the functionality that it provides. It willbe clear to persons of ordinary skill in the data communications fieldhow to fabricate each module with either special purpose hardware orwith appropriately programmed general purpose hardware. It is preferredthat the illustrative embodiment be constructed with either electrical,optical or electro-optical technologies. It will be clear to persons ofordinary skill how to practice the instant invention or so much of it asis of use.

The first module 217 in the embodiment advantageously receives andaccumulates one or more cells from the virtual circuit until a trailercell is received. When a trailer cell is received, the trailer cell andall (if any) of the user-data cells accumulated since the receipt of thelast trailer cell are assembled, in the order received, into a"candidate frame".

The second module 219 in FIG. 2 in the embodiment advantageouslyvalidates the integrity of the trailer. As shown at 619 in FIG. 6,validating the integrity of the trailer advantageously comprises threeoperations: (1) locating the trailer within the candidate frame, (2)checking the trailer for transmission errors and (3) parsing thetrailer.

The second module locates the trailer within the candidate cell byadvantageously scanning the trailer cell in eight byte steps beginningfrom the end of the cell forward to the front of the cell until anon-zero byte is found. Since the last byte of data in the CRC-trailerwas advantageously modified, during the framing process, to be alwaysnon-zero, this non-zero byte serves as a delimiter for finding the endof the trailer.

When no non-zero byte is found within the last 41 bytes of the cell, atransmission error has occurred. In such a case, the candidate frame isdiscarded, the process of reclaiming the packet is terminated, and amessage is transmitted to Bob indicating that an error has occurred inthe transmission of a packet from Alice.

When the trailer is located, the second module checks the trailer fortransmission errors by feeding the signals within trailer (i.e., theuser-data-length, the sequence-identifier and the CRC-trailer fields)into a Cyclic-Redundancy-Code mechanism with the same generatorpolynomial used for generating the data within the CRC-trailer field.

When the result of the Cyclic-Redundancy-Code check indicates that thedata within the trailer is corrupted, the candidate frame is discarded,the process of reclaiming the packet is terminated and a message istransmitted to Bob indicating that an error has occurred.

When the integrity of the trailer is validated, the second module parsesthe trailer. The trailer is advantageously eight bytes long and isparsed as follows: the first three bytes of the trailer contain thesequence-identifier and the next two bytes contain the user-data-lengthfield.

The third module 221 in the embodiment validates the integrity of thepacket (i.e., the user-data within the user-data field). As shown at 721in FIG. 7, the operation of checking the integrity of the user-dataadvantageously comprises four steps: (1) checking the consistency of theuser-data-length and the number of data cells in candidate frame, (2)verifying the sequence-identifier, (3) verifying the user-data-length,and (4) checking the integrity of the user-data.

When the value of the user-data-length field is 36 or less (meaning thatthe candidate frame fits within one cell), the data cells within thecandidate frame are discarded and the candidate frame is, from thispoint forward in the reclaiming process, considered to contain only thetrailer cell.

The third module in the embodiment advantageously checks the candidateframe's sequence-identifier against Bob's record of the previous frame'ssequence-identifier to assure that the candidate frame'ssequence-identifier is one more (mod 2²⁴) than that of the previousframe. Regardless of the result, Bob's record of the previous frame'ssequence-identifier is updated with the candidate frame'ssequence-identifier. When, however, the candidate frame's sequencenumber is out of sequence and the user-data-length is 37 or more, thecandidate frame is discarded, the process of reclaiming the packet isterminated and a message is transmitted to Bob indicating that an errorhas occurred.

Next, the user-data-length is checked to assure that it is less than orequal to the total number of bytes in the candidate frame's user-dataand padding fields. When the condition is not satisfied, the candidateframe is discarded, the process of reclaiming the packet is terminated,and a message is transmitted to Bob indicating that an error hasoccurred. Otherwise, the user-data-length is used to parse the user-datafrom the padding.

The user-data is checked for transmission error by feeding the user-dataand the padding signals and the CRC-data into a Cyclic-Redundancy-Codemechanism with the generator polynomial used for generating the datawithin the CRC-data field. When the result of the Cyclic-Redundancy-Codemechanism indicates that the user-data and the padding are corrupted,the candidate frame is discarded, the process of reclaiming the packetis terminated and a message is transmitted to Bob indicating that anerror has occurred. Otherwise, the user-data has been received, completeand uncorrupted and is thereafter considered the original packet whichis transmitted to Bob 203.

6. Examples

6.1 A Packet Which Fits Into a Single Cell

A sample frame containing the packet "Hello world<CR><LF>" (13 bytes)appears in Table 1.

                  TABLE 1                                                         ______________________________________                                        48  65    6C     6C  user-data "Hell"                                         6F  20    77     6F  user-data "o wo"                                         72  6C    64     0D  user-data "rld<CR>"                                      0A  00    00     00  user-data "<LF>"; 3 padding bytes                        00  00    00     00  4 additional padding bytes                               4C  0C    60     C9  CRC-data=4C0C60C9                                        12  34    56     00  sequence-number=123456                                   0D  2D    AE     0D  user-data-length=13; CRC-trailer=2DAE0D                  00  00    00     00  16 fill bytes                                            00  00    00     00                                                           00  00    00     00                                                           00  00    00     00                                                           ______________________________________                                    

6.2. A Packet Which Requires Multiple Cells

A sample frame containing the packet "The fault, dear Brutus, is not inour stars,<CR><LF>But in ourselves . . . <CR><LF>" (67 bytes) appears inFig. L.

                  TABLE 2                                                         ______________________________________                                        54  68     65    20   user-data "The"                                         66  68     65    20   user-data "faul"                                        74  2C     20    64   user-data "t, d"                                        65  61     72    20   user-data "ear"                                         42  72     75    74   user-data "Brut"                                        75  73     2C    20   user-data "us,"                                         69  73     20    6E   user-data "is n"                                        6F  74     20    69   user-data "ot i"                                        6E  20     6F    75   user-data "n ou"                                        72  20     73    74   user-data "r st"                                        61  72     73    2C   user-data "ars,"                                        0D  0A     42    75   user-data "<CR><LF>Bu"                                  74  20     69    6E   user-data "t in"                                        20  6F     75    72   user-data "our"                                         73  65     6C    76   user-data "selv"                                        65  73     2E    2E   user-data "es.."                                        2E  0D     0A    00   user-data ".<CR><LF>"; 1 padding byte                   37  C7     99    E0   CRC-data=37C799E0                                       00  4E     21    00   sequence-number=200001                                  43  4C     14    7D   user-data-length=67; CRC-trailer=4C147D                 00  00     00    00   16 fill bytes                                           00  00     00    00                                                           00  00     00    00                                                           00  00     00    00                                                           ______________________________________                                         ##SPC1##

We claim:
 1. An apparatus for generating at least two ordered sets ofoutput signals based on an ordered set of input signals, said apparatuscomprising:means for forming, for each ordered set of input signals, (1)an ordered set of data signals representative of said ordered set ofinput signals; (2) a first ordered set of error detection signals basedon said ordered set of data signals; (3) an ordered set of sequenceidentifier signals identifying said ordered sets of output signals; (4)an ordered set of length signals based on a length of said ordered setof data signals; (5) a second ordered set of error detection signalsbased on said ordered set of sequence identifier signals and saidordered set of length signals; and means for generating said orderedsets of output signals such that said ordered set of sequence identifiersignals, said ordered set of length signals and said second ordered setof error detection signals are contained within a single ordered set ofoutput signals and at least a portion of said ordered set of datasignals are contained in another ordered set of output signals.
 2. Theapparatus of claim 1 wherein said means for forming furthercomprises:means for developing said first ordered set of error detectionsignals; means for developing said ordered set of length signals; andmeans for developing said second ordered set of error detection signals.3. The apparatus of claim 2 wherein said means for developing said firstordered set of error detection signals further comprises means fordeveloping a cyclic-redundancy-code.
 4. The apparatus of claim 3 whereinsaid means for developing a cyclic-redundancy-code comprises means forgenerating said cyclic-redundancy-code based on a generator polynomialequal to g(x)=x³² +x³¹ +x⁴ +x³ +x+1.
 5. The apparatus of claim 2 whereinsaid means for developing said second ordered set of error detectionsignals further comprises means for developing a cyclic-redundancy-code.6. The apparatus of claim 5 wherein said means for developing acyclic-redundancy-code is defined by a generator polynomial equal tog(x)=x²³ +x²² +x² +1.
 7. The apparatus of claim 1 further comprisingmeans for identifying each ordered set of output signals as being one oftwo possible types of ordered sets of output signals.
 8. An apparatusfor generating an ordered set of output signals based on at least twoordered sets of input signals, said apparatus comprising:means forforming an ordered set of candidate signals from at least two of saidordered sets of input signals such that a single ordered set of inputsignals contains an ordered set of sequence identifier signals, anordered set of length signals and a first ordered set of error detectionsignals; and means for parsing said ordered set of candidate signals,said ordered set of candidate signals comprising:(1) an ordered set ofdata signals representative of said ordered set of output signals; (2) asecond ordered set of error detection signals based on said ordered setof data signals; (3) said ordered set of sequence identifier signalsidentifying said ordered set of candidate signals; (4) said ordered setof length signals based on a length of said ordered set of data signals;(5) said first ordered set of error detection signals based on saidordered set of sequence identifier signals and said ordered set oflength signals.
 9. The apparatus of claim 8 wherein said means forparsing further comprises means for checking said ordered set of datasignals with a cyclic-redundancy-code.
 10. The apparatus of claim 9wherein said means for checking is defined by a generator polynomialequal to g(x)=x³² +x³¹ +x⁴ +x³ +x+1.
 11. The apparatus of claim 8wherein said means for parsing further comprises means for checking saidordered set of sequence signals and said ordered set of length signalswith a cyclic-redundancy-code.
 12. The apparatus of claim 11 whereinsaid means for checking is defined by a generator polynomial equal tog(x)=x²³ +x²² +x² +1.
 13. The apparatus of claim 8 further comprisingmeans for identifying each ordered set of input signals as being one oftwo possible types of ordered sets of input signals.
 14. A method forgenerating at lest two ordered sets of output signals based on anordered set of input signals, said method comprising the stepsof:forming an augmented ordered set of input signals, said augmentedordered set of input signals comprising:(1) an ordered set of datasignals representative of said ordered set of input signals; (2) a firstordered set of error detection signals based on said ordered set of datasignals; (3) an ordered set of sequence identifier signals identifyingsaid augmented ordered set of input signals; (4) an ordered set oflength signals based on a length of said ordered set of data signals;(5) a second ordered set of error detection signals based on saidordered set of sequence identifier signals, and said ordered set oflength signals; and generating said ordered sets of output signals basedon said augmented ordered set of input signals such that said orderedset of sequence identifier signals, said ordered set of length signalsand said second ordered set of error detection signals are containedwithin a first ordered set of output signals and at least a portion ofsaid ordered set of data signals are contained in a different orderedset of output signals.
 15. The method of claim 14 wherein said step offorming further comprises the steps of:developing said first ordered setof error detection signals; developing said ordered set of lengthsignals; and developing said second ordered set of error detectionsignals.
 16. The method of claim 15 wherein said step of developing saidfirst ordered set of error detection signals further comprises the stepof developing a cyclic-redundancy-code.
 17. The method of claim 16wherein said step of developing a cyclic-redundancy-code is defined by agenerator polynomial equal to g(x)=x³² +x³¹ +x⁴ +x³ +x+1.
 18. The methodof claim 15 wherein said step of developing said second ordered set oferror detection signals further comprises developing acyclic-redundancy-code.
 19. The method of claim 18 wherein said step ofdeveloping a cyclic-redundancy-code is defined by a generator polynomialequal to g(x)=x²³ +x²² +x² +1.
 20. The method of claim 14 furthercomprising the step of identifying each ordered set of output signals asbeing one of two possible types of ordered sets of output signals.
 21. Amethod for generating an ordered set of output signals based on at leasttwo ordered sets of input signals, said method comprising the stepsof:forming a ordered set of candidate signals from at least two of saidordered sets of input signals such that a single ordered set of inputsignals contains an ordered set of sequence identifier signals, anordered set of length signals and a first ordered set of error detectionsignals; and parsing said ordered set of candidate signals, said orderedset of candidate signals comprising:(1) an ordered set of data signalsrepresentative of said ordered set of output signals; (2) a secondordered set of error detection signals based on said ordered set of datasignals; (3) said ordered set of sequence identifier signals identifyingsaid ordered set of candidate signals; (4) said ordered set of lengthsignals based on a length of said ordered set of data signals; (5) saidfirst ordered set of error detection signals based on said ordered setof sequence signals and said ordered set of length signals.
 22. Themethod of claim 21 wherein said step of parsing further comprises thestep of checking the integrity of said ordered set of data signals witha cyclic-redundancy-code.
 23. The method of claim 22 wherein said stepof checking is defined by a generator polynomial equal to g(x)=x³² +x³¹+x⁴ +x³ +x+1.
 24. The method of claim 21 wherein said step of parsingfurther comprises the step of checking the integrity of said ordered setof sequence signals and said ordered set of length signals with acyclic-redundancy-code.
 25. The method of claim 24 wherein said step ofchecking is defined by a generator polynomial equal to g(x)=x²³ +x²² +x²+1.
 26. The method of claim 21 further comprising the step ofidentifying each ordered set of input signals as being one of twopossible types of ordered sets of input signals.
 27. An apparatus forframing a packet, said apparatus comprising:means for forming a framebased on said packet, said frame comprising: (1) said packet including afirst error-detection signal based on said packet, (2) asequence-identifier identifying said frame, and (3) a seconderror-detection signal based on said sequence-identifier; and means forsegmenting said frame into at least two cells such that saidsequence-identifier and said error-detection signal are placed into onecell and at least a portion of said packet is placed into at least oneadditional cell.
 28. The apparatus of claim 27 wherein said framefurther comprises: (4) a length-indicator based on of the length of saidpacket.
 29. The apparatus of claim 28 wherein said seconderror-detection signal is also based on said length-indicator.
 30. Theapparatus of claim 29 wherein said frame is segmented such that saidsequence-identifier, said error-detection signal and saidlength-indicator are placed into a single cell.
 31. An apparatus forreclaiming a packet, said apparatus comprising:means for forming acandidate frame from at least two cells, such that one of said cells insaid candidate frame comprises a sequence-identifier and a firsterror-detection signal; and means for parsing said candidate frameinto(1) said packet, including a second error-detection signal based onsaid packet, (2) said sequence-identifier, which identifies saidcandidate frame, and (3) said first error-detection signal, which isbased on said sequence-identifier.
 32. The apparatus of claim 31 whereinsaid candidate frame further comprises: (4) a length-indicator based onof the length of said packet.
 33. The apparatus of claim 32 wherein saidfirst error-detection signal is also based on said length-indicator. 34.The apparatus of claim 33 wherein one of said cells in said candidateframe comprises said sequence-identifier, said error-detection signaland said length-indicator.
 35. A method of framing a packet comprisingthe steps of:forming a frame based on said packet, said framecomprising:(1) said packet, including a first error-detection signalbased on said packet, (2) a sequence-identifier identifying said frame,and (3) a second error-detection signal based on saidsequence-identifier; and segmenting said frame into at least two cellssuch that said sequence-identifier and said second error-detectionsignal are placed into one cell.
 36. The method of claim 35 wherein saidframe further comprises: (4) a length-indicator based on of the lengthof said packet.
 37. The method of claim 36 wherein said seconderror-detection signal is also based on said length-indicator.
 38. Themethod of claim 37 wherein said frame is segmented such that saidsequence-identifier, said error-detection signal and saidlength-indicator are placed into a single cell.
 39. A method ofreclaiming a packet comprising the steps of:forming a candidate framefrom at least two cells, such that one of said cells in said candidateframe comprises a sequence-identifier and a first error-detectionsignal; and parsing said candidate frame into(1) said packet, includinga second error-detection signal based on said packet, (2) saidsequence-identifier, which identifies said candidate frame, and (3) saidfirst error-detection signal, which is based on saidsequence-identifier.
 40. The method of claim 39 wherein said candidateframe further comprises: (4) a length-indicator based on of the lengthof said packet.
 41. The method of claim 40 wherein said seconderror-detection signal is also based on said length-indicator.
 42. Themethod of claim 41 wherein one of said cells in said candidate framecomprises said sequence-identifier, said error-detection signal and saidlength-indicator.